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angelo
Posted: February 14, 2016 11:30 am
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Dear all,

need a confirmation if i am thinking right, or if i am wrong.

This chunk of schematic is from a freescale development board.
It is a reset circuit, done by:
- classic simple R/C reset to cpu
- push button in parallel to C for manual reset
- additional reset (OSBDM_nRESET) coming from a debugger/programmer, connected directly
(0 ohm bridge) to cpu reset pin.

Here on this third point i have doubts: following this schematic, if i close the push button, i force the programmer reset output (OSBDM_nRESET) to GND, and this seems not correct / sane, i could damage the programmer.

http://5.39.81.93/~angelo/reset.png

for this purpose, i am thinking to use instead an AND port. What do you think ?
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Sch3mat1c
Posted: February 14, 2016 02:34 pm
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Does the programmer documentation say if the reset pin is open collector?

Tim


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angelo
Posted: February 14, 2016 02:41 pm
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Hi Tim,

mainly i refer to "my" programmer, since this is my use case.
My programmer chip (that is not the one sold from Freescale but a cheaper, and maybe less protected one) is based on a MC68HC908, with some unknown fw (maybe readable) programmed in, reset comes out directly from it, so probably that pin is set fixed as output, maybe it is put in hi-z when he is not resetting.

There is always the unfortunate case that i reset with push-button while debugger is resetting too.
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Sch3mat1c
Posted: February 14, 2016 02:43 pm
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Well, then measure it. Check if it really only uses two states, Hi-Z and '0', or if you need to use a diode to protect it during a '1' state. thumbsup.gif

Tim


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angelo
Posted: February 14, 2016 04:09 pm
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ok measured

this his how the reset behave

http://sysam.it/~angelo/TEK00045.PNG

Seems like open-collector to 0

Btw, how can i be sure the capacitor don't delay the reset from the programmer ?
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Sch3mat1c
Posted: February 14, 2016 07:01 pm
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Well, you can test that too, and find out...

Or, what's the programmer's delay between deasserting RESET and beginning data?

Or, it's the kind of programming method where the processor is held in reset during programming, in which case it doesn't even matter.

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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